A billion-yuan stadium in Inner Mongolia has collapsed, the latest in a string of 'tofu' projects on the mainland sparking concerns about building safety.

The steel structure of the 30,000-seat main stadium for the International Naadam Festival in Ordos collapsed six months after it was used for the first time.

A mainland journalist taking pictures of the collapsed stadium yesterday from a distant highway was taken away by police. The site has been sealed off for about three weeks since the collapse.

Confirming the structure's collapse, Ordos propaganda chief Su Jianrong said the site had been sealed for further investigation. 'I have no more to tell you except to confirm the collapse of the Naadam Festival's main stadium about 20 days ago. I do not have information as to the cause of the incident,' Su said.

According to Beijing-based news portal Zijing.org, the stadium cost about one billion yuan (HK$1.18 billion) to build. Construction of the stadium began in December 2009.

More than 30,000 tonnes of steel were used for the stadium, about two-thirds of the amount used to build Beijing's 'Bird's Nest' for the 2008 Olympic Games.

The collapsed stadium was used as the venue for horse races during the first Ordos International Naadam Festival from August 11 to 18. Naadam is a traditional holiday dating back 700 years featuring horse racing, archery and wrestling. The stadium was meant to be finished by June but Xinhua reported in July only 35 per cent of the overall project had been completed and work was continuing.

Local media are speculating that the stadium collapsed because work was rushed so it could be finished before the festival.

It is not the first time poor quality construction on the mainland has sparked concern.

In 2008, thousands of students were killed when their classrooms were reduced to rubble in just a few minutes by a magnitude-8 earthquake in Sichuan . Many blame the high death toll on the shoddy materials used in the school buildings and design flaws.

Before that, 64 people died in August 2007 when the 328-metre Tuojiang bridge in Hunan province collapsed as workers were dismantling its steel scaffolding. In December 2009, substantial cracks were found in Nanjing's 50 million yuan Hanzhongmen Bridge.